This invention relates to shaped thermoplastic articles. More particularly, this invention relates to a composite sheet consisting of: thermoplastic resin, short reinforcing fibers, long or continuous glass fibers, and a particulate loading agent which may be readily formed in conventional stamping apparatus into smooth surfaced, shaped objects. Shaping of the composite sheets may be effected by maintaining the shaping apparatus at temperatures between room temperature and 200.degree. C. while preheating the sheet to a temperature above the softening point of the polymer. Under these operating conditions the total cycle time may be retained below about 30-40 seconds on even extremely large shaped parts.
It is known that many thermoplastic polymers can be formed into shaped objects at ambient temperatures by various sheet metal forming techniques such as deep drawing, stretch forming, stamping, forging, cold extrusion, etc. These cold forming processes are economically very attractive because it is possible to achieve rapid production rates, e.g., rates that exceed one item a second or even faster. Unfortunately, the thermoplastics prepared by such methods exhibit deficiencies in their properties, such as built-in strains and stresses, non-uniformity in wall thickness, tendency towards stress-cracking, poor dimensional stability, low modulus and strength, etc.
In order to overcome these deficiencies, processes such as those described in U.S. Pat. No. 3,621,092 and copending U.S. applications Ser. No. 195,450, filed on Jan. 11, 1971 and Ser. No. 268,400, filed on July 3, 1972 involving the rapid stamping of a glass fiber mat reinforced thermoplastic have been developed
Generally in this process, a thermoplastic sheet, reinforced with a glass mat type of reinforcement is pre-heated in an oven to above the softening point of the resin. The heated blank is transferred to the matched dies of a stamping press, and stamped in a modified mechanical or rapid-closing hydraulic press, or the like. The formed part is then removed or ejected from the matched dies. The residence time in the mold is 20-80 seconds or less. The forming process can be separated into the following distinct stages.
1. Pre-heat of sheet or blank PA1 2. Transfer to the rapid-stamping press PA1 3. Stamping under pressure for a predetermined time PA1 4. Removal or ejection from the press PA1 a. from about 25 to 65% of a synthetic thermoplastic resin, PA1 b. from about 20 to 50% of a particulate filler, PA1 c. from about 15 to 45% of short glass reinforcing fibers having a length not greater than 0.85 inch, at least 50% of said fibers being aligned substantially parallel to the plane of the sheet, and PA1 d. from about 2 to 15% of long glass fibers having a length of about 11/2 inches to continuous and being substantially below the exterior surface of the composite sheet.
In stage 1, the composite sheet or blank is heated in a radiant, dielectric, infrared, convection, or vacuum oven or combination of ovens or similar heating source to a temperature above the melting point but below the decomposition point of the thermoplastic resin component of the blank.
In stage 2, the hot blank is transferred to a mold placed in a stamping press, wherein the mold or set of dies can impart the desired configuration of the final product to the sheet or blank. The mold is maintained at a temperature between room temperature (23.degree. C.) and approximately 160.degree. C. or more, depending on the polymer constituent of the sheet and upon the desired stamping characteristics.
In stage 3, the press is rapidly closed for a period of time sufficient to cause the blank to conform to the exact shape of the mold and to cool and/or crystallize sufficiently to allow part removal without distortion.
In stage 4, the molded article is removed from the mold.
It will be recognized that stamping parameters such as pressure requirements, residence time in the mold, pre-heat temperature, mold temperature, etc., are dependent upon the sheet composition, thickness, part complexity, etc. as described for example in the above mentioned U.S. patent applications Ser. Nos. 195,450 and 268,400.
A process of this kind has many advantages over conventional forming methods. (a) The cycle times are extremely fast, with a cycle time of 10-30 seconds per part often being realizable. (b) Standard sheet-metal stamping, hydraulic or mechanical stamping presses are usable, although slight modifications may be required in the clutch assembly in order to obtain the desired pressure cycle. (c) The shaped articles may have complex and non-planar configurations. (d) Variable thicknesses relative to the initial sheet or blank thickness can be achieved during stamping, or by "building up" the blank thickness prior to stamping. (e) Holes and notches can be formed during stamping or in post-forming operations.
Glass mat reinforced sheets of this kind as provided heretofore also suffer from certain disadvantages. Specifically the following deficiencies have been encountered in prior art compositions:
Surface Finish -- Prior art compositions usually contain 30 to 60% by weight of glass mat. Such an amount is required in order to impart high mechanical strength properties, high modulus, and sheet integrity during the pre-heat and transfer stages of the rapid stamping cycle. During the pre-heat stage, the glass mat reinforced sheet is supported on thin rods, point-supports or in edge clamps, in order to allow equally thorough, rapid and uniform heating of both sides of the sheet. It will be apparent that the heated sheet, to permit it to be transferred to the stamping apparatus, must remain rigid and strong during this heating stage bearing in mind that temperatures in excess of the melting temperature are often needed to soften the interior of the thermoplastic sheet. Especially at the surface of the sheet, temperatures up to 20.degree. C.-50.degree. C. or more above the melt temperature are encountered, and the polymer becomes substantially fluid. The glass mat functions predominantly in retaining the integrity and strength of the sheet during the pre-heat stage. This function will be considered further hereinafter.
During the transfer stage, the hot or semi-molten thermoplastic sheet is transferred mechanically from the pre-heat oven or other heating environment to the stamping press. One method of transferring the hot thermoplastic sheet is by clamping the sheet along the edges and thus moving the clamped sheet to the stamping press. This method of sheet transfer does not interfere with or disrupt the integrity of the center of the heated sheet but it is essential that the sheet be strong enough during the transfer stage to support its own weight when clamped along the edges. An unreinforced sheet processed in this manner, for example, virtually disintegrates during the pre-heat and/or transfer stages because of insufficient integral strength in the melt when heated above the melting point. Moreover, even a sheet reinforced solely with short glass fibers, i.e., glass fiber 1/8-1/2 inch long or shorter, has insufficient strength to remain as an integral sheet during the pre-heat and transfer stages. Additionally for similar reasons, a thermoplastic resin sheet reinforced with an insufficient amount of glass mat will not remain intact during the pre-heat stage. For example, a thermoplastic resin sheet reinforced with only 5-20% of glass mat, when the resin is heated above its melting point, will drip and lose resin during the pre-heat stage, and may actually disintegrate into several non-associated sections during the transfer stage.
Thus, a substantial amount of glass mat is needed not only to yield good mechanical strength properties in the final stamped part, but also to retain sheet integrity during the preheat and transfer stages of the rapid stamping process. However, a high concentration of glass mat of the quantity needed to provide the necessary mechanical strength and to retain the integrity of the sheet when it is heated leads to a poor surface finish on the final, stamped part. For application such as automotive exterior parts (fenders, hoods), appliance housings, furniture components, etc., a perfectly smooth, imperfection free surface is mandatory. The measured depth of surface imperfections should be no more than 50-500 microinches (10.sup.-.sup.6 inch) when measured using a standard Bendix Corp. Micro-corder, profilometer or similar stylus-type profile indicator.
A high concentration of glass mat near the surface of a stamped part ordinarily leads to imperfections larger than this limiting value. The cause for this is several-fold. First, glass fiber mats are not dispersed or filamentized into the thermoplastic resin matrix during the stamping cycle. This is to be distinguished from thermoplastic fabricating processes such as injection molding or extrusion, in which the glass fiber strands are broken up into the individual filaments and dispersed into the matrix. Since glass fiber strands normally consist of approximately 30-400 or more filaments per strand, and each individual filament is 0.0003 to 0.0008 inches in diameter, the breaking up and dispersion of these filaments is an important factor in obtaining smooth, fiber-free surfaces in injection molded or extruded objects. In stamping of glass fiber mat reinforced thermoplastics, however, extreme shear and mechanical working is not encountered. Rather, compressive stresses and mild, somewhat limited, translatory motion of the reinforced thermoplastic is encountered. This results in flow of the entire filament bundle (or strand) of glass fiber, rather than breaking up of the strand into the individual filaments. A bundle of glass fibers, or equivalently, a section of glass mat, embedded in the thermoplastic resin, leads to surface imperfection such as "hills and valleys" or "orange peel" effects, roughness, waviness, ripples, and in general, imperfections greater than 50-500, microinches in depth, as will be explained below. The higher the concentration of the reinforcing glass mat, the greater is the likelihood of occurrence of the detrimental surface imperfections. Thus, a primary cause of surface imperfections in stamped, glass mat reinforced thermoplastic sheets is the reinforcing glass mat itself.
Lack of Uniformity -- Another deficiency encountered in stampable reinforced sheet compositions of the type with which the present invention is concerned is non-uniformity of properties in the finished stamped object. This non-uniformity is generally caused by two discrete factors. First, the composition of the glass mat reinforced sheet is homogeneous only on a macro-scale. On a micro-scale, the reinforced sheet is seen to consist of a web-like glass mat, with a separate resin phase forming a distinct region within the whole structure. This resin-rich region is necessarily mechanically weak because of the absence of reinforcing glass fibers. This distinct two-phase characteristic is not encountered in more conventional fabricating methods, such as injection molding, since, as described above, a more homogeneous glass fiber dispersion is obtained in the conventional processing techniques.
The second factor causing non-uniformity of properties is the flow of the resin during the stamping stage, especially flow over, or into, highly irregular corners, edges, ribs, bosses, inserts, and the like. Such flow often serves to separate the matrix resin and reinforcement phases of the composite, again leading to non-uniformity in composition and properties of the article. Separation of the glass fiber reinforcement from the matrix resin is often known as "bridging", and occurs when a molded section, attachments, or contour causes preferential flow and/or separation of the matrix or reinforcement. For example, in the molding or stamping of a part containing a thin raised section (see FIG. 8), the fibrous reinforcement will not readily flow into such a thin section since some fibers will naturally form a "bridge" or obstruction across the entrance to the thin section, resulting in preferential flow of resin into the section.
If the dimension of the thin section or entrance to such a section is large when compared to the length of the fibrous reinforcement, bridging will not occur. In the case of materials reinforced solely with very long or continuous fibers, bridging will be very pronounced since the fiber length is inherently large when compared to sections of a complex part. Thus, prior art compositions often suffer from significant non-uniformity of properties in all but extremely simple, or nearly-flat objects.
Lamination Processing -- A desirable method of continuously producing the sheet compositions formable by the rapid stamping technique herein described is by a continuous extrusion/lamination scheme. Using such a method, two plies of thermoplastic sheeting are extruded or similarly produced; glass mat or its equivalent is then fed in between the two plies; the proper combination of heat and pressure (supplied by laminating rolls or the like) is then used to fuse and laminate the two sheets and the glass fiber web, mat, or the like together into an integral sheet construction.
If a high concentration of glass mat is needed in the composite sheet, as in the present invention, a continuous mode of fabrication presents significant practical difficulties. For example, at a glass mat concentration of 40% by weight, a thermoplastic sheet may require 3-6 ounces/ft.sup.2 of glass mat. Such a weight of glass mat would have a thickness prior to compression or lamination of 0.100 inch to 0.200 inch. After compression and lamination, the total sheet thickness (i.e., two thermoplastic plies plus the central glass mat) would be in the range of 0.085 to 0.105 inch. It is clear that the large initial thickness of glass mat complicates the continuous lamination scheme, since compression and impregnation of a large thickness of glass fibers is difficult to accomplish thoroughly.